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P73
Shatil Amin
March 27th 2003
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..Content
I. Structure and Function
II. Regulation
III. Is it involved in human cancers?
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Early Findings
• Related to p53 tumor suppressor family
• Activated by DNA damage
• Mediates GI/S cell cycle arrest and apoptosis
• p73 is a transcription factor– Target genes: BAX apoptosis
WAF1(p21)(p21) Cell cycle arrest
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DNA damage
p73
TARGET GENES Apoptosis Growth Arrest
P21 BAX
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TP73 gene many different mRnas
• TP73 gene produces 2 classes of isoforms:– TAp73 isoform (transcriptionally active ,
apoptotic/growth inhibitory activity)
– ∆Np73 isoform
• ∆Np73 isoform lacks transcriptional activity
– Amino truncated
– Controlled by alternate promoter in the same gene
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TP73 gene
Promoter 1 TAp73 isoforms (transcriptionally active…antiapoptotic and growth inhibitory activity)
Promoter 2 Np73 isoforms (NO transcriptional activity)
One gene, but two different proteins under the control of two distinct promoters
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∆Np73 isoform is a dominant negative regulator of p53/TAp73
• Inhibits p53 and p73 – Competition for binding– Oligimerization
• Oncogenic properties !!
• p53 and TAp73 activate Np73 Dominant negative feedback loop
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How does NAp73 inhibit
p53 and TAp73?
Tight regulation via dominant negative
feedback loop!
-Oligimerization
-Competition for binding
sites
**One gene: 2 products that are functionally antagonistic**
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Involvement in Cancer?
• Ip36 locus commonly deleted in tumors– Is p73 a tumor suppressor ?
• Tp73 mutations rare in human primary tumors– Fewer than .5%
• Tp73 knockout mice don’t produce tumors
So…this evidence suggests it’s not a classical tumor suppressor
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Complications in assessing the role of p73 in tumorigenesis
• Tp73 encodes two functionally opposing proteins: – An in vitro tumor suppressor (TAp73) and a putative
oncogene (∆Np73)
• Mutations may affect both TAp73 and ∆Np73 together– Deletions
• Abrogate both in vitro growth inhibitory and oncogenic activity…..(no net effect!)
• Need to discriminate between TAp73 and ∆Np73 isoforms !!
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Experiments that Discriminate between TAp73 and ∆Np73
Variants of ∆Np73 (with anti-apoptotic activity) overexpressed in breast cancer cell lines, ovarian cancer, vulval cancer, and neuroblastic tumors
Np73 isoform
Ng SW et.al
Used RTPCR
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• ∆Np73 expression strong adverse prognostic indicator in Neuroblastoma – No Np73 expression = 80% survival– Overexpress Np73 = none survived
• ∆Np73 function (blocking p53 and TAp73 mediated apoptosis) is key to development of tumor
• Mutation/inactivation of entire gene does not necessarily lead to cancer
• TA:∆N ratio is what may be altered in cancer !– Regulating respective promoters (mythylation)
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Review of Main Points
• Tp73 gene: two functionally different proteins – TAp73: stimulates apoptosis and cell cycle arrest in
response to DNA damage– ∆Nap73: negative regulator of p53 and TAp73 with
oncogenic properties
• Enhanced expression of Np73 form associated with cancer
• Future Research: assessing TA:Np73 ratios in cancer